1. Field of the Invention
The present invention relates to a device for detecting the remaining quantity of sheets by using a distance measuring sensor, and to an image forming apparatus including such a device.
2. Description of the Related Art
In a paper feeding device including a sheet setting part for setting sheets and in which the sheets on the sheet setting part are sequentially fed, there are types that are placed with a remaining sheet quantity detection device using a distance measuring sensor.
The distance measuring sensor measures the distance between itself and the sheet at the uppermost position of the sheet setting part in order to detect the remaining sheet quantity in the sheet setting part.
FIG. 10 is a diagram showing the output characteristics of the distance measuring sensor. As shown in FIG. 10, the voltage value (level value) Vout obtained by the distance measuring sensor sequentially increases as the distance L becomes longer when the distance L between the distance measuring sensor and the object to be measured is less than 2 cm. In addition, the voltage value Vout becomes the peak when the distance L is 2 cm, and when the distance L thereafter exceeds 2 cm, the voltage value Vout sequentially decreases as the distance L increases.
Based on the foregoing output characteristics, for example, when the voltage value Vout is 2.0 V, it can be seen that the distance of “1 cm” and the distance of “4 cm” are obtained as the distance corresponding to the voltage value Vout.
Meanwhile, in order for the remaining sheet quantity detection device to detect the remaining sheet quantity by using the distance measured by the distance measuring sensor, one distance needs to be set forth as the distance corresponding to the voltage value obtained by the distance measuring sensor.
Thus, with the distance measuring sensor, the distance where the voltage value Vout becomes a peak is set as the minimum detection distance its terms of its specification (2.0 cm in the case of the distance measuring sensor with the output characteristics of FIG. 10). In addition, the distance measuring sensor is disposed so that the minimum distance between the distance measuring sensor and the object to be measured exceeds the minimum detection distance so that the distance measuring sensor will not measure a distance that is less than the minimum detection distance.
FIG. 11 is a diagram showing a stocker for housing the sheets. Note that, in FIG. 11, reference numeral 331 shows the lower face of the stocker 33. With the stocker 33, a first tray 334 and a second tray 335 are filled with the predetermined maximum number of sheets SH in a state where the upper face of the tray is positioned at the lowermost position PL, and are thereafter pushed upward toward the uppermost position PH as the sheet on the upper face of the tray is ejected.
Consequently, as the sheets on the upper face of the tray are ejected, the distance between the tray upper face and the distance measuring sensors S1 and S2 will sequentially decrease toward the minimum distance L(min).
Even with this kind of stocker 33, the distance measuring sensor is disposed so that the minimum distance L(min) exceeds the minimum detection distance of the distance measuring sensor so as to obtain one distance corresponding to the voltage value obtained by the distance measuring sensors S1 and S2.
Accordingly, since the distance measuring sensor needs to be positioned so that the minimum distance between the distance measuring sensor and the object to be measured exceeds the minimum detection distance of the distance measuring sensor, there are restrictions in the positioning of the distance measuring sensor.
Moreover, since the minimum detection distance tends to become longer as the measurable distance becomes longer with a distance measuring sensor, the positioning of the distance measuring sensor becomes further restricted.